This invention is concerned with surface protection of halide solids. In particular, this invention is concerned with the surface protection of water soluble halide solids for use in optical components in infrared systems. A thallium iodide has been demonstrated as an excellent protective high index coating material for potassium chloride laser windows. TlI has a high refractive index and an optical absorption of less than 1 cm.sup.-1 at 10.6 microns. The protective quality of TlI lies in its insolubility and remarkable adhesion to potassium chloride.
Other patents assigned to the same assignee as the present application and dealing with protective coatings on water soluble halide optical elements are U.S. Pat. Nos. 4,009,300 and 3,959,548.
In the past thallium iodide films vacuum deposited on potassium chloride substrates, in thickness about 1 to 2 microns sufficient for infrared antireflection coatings, have been very cloudy due to the scatter of visible light by a birefringement epitaxial thallium iodide microstructure.
The adhesion referred to is due, in part, to the epitaxial growth of TlI on KCl. TlI microstructures resulting from different epitaxial growth habits are responsible for the optical scattering observed in films grown on certain KCl orientations.
We have found that the alignment of the TlI b.sub.o axis parallel to the KCl &lt;110&gt; direction nearest the substrate surface determines the epitaxial growth habit. On the precise KCl (100) orientation, for example two KCl &lt;110&gt; directions lie in the surface. This causes the TlI to nucleate in two orientations rotated 90.degree. from one another. This unusual microstructure results in scattering by the diffraction of light passing from one biaxially birefringement orientation to another.
More specifically, the cloudiness in films on these crystals occurs in specific crystallographic regions. A lens-shaped crystal, for example, is cloudy along the two KCl &lt;100&gt; directions projected from the KCl &lt;100&gt; pole. These cloudy regions contain the microstructure mentioned previously and divide the lens into four quadrants. The entire lens was studied between crossed polarizers. With the KCl &lt;100&gt; directions parallel to the polarizer axes only the microstructural boundaries are extinguished. As the crystal is rotated with respect to the polarizers the entire film is extinguished when the KCl &lt;110&gt; directions become parallel to the polarizer axes. This implies that the TlI contains, at most, two orientations, both of which have their optic axes parallel to the two KCl &lt;110&gt; directions. The fact that boundaries appear means that there are indeed two TlI orientations.
X-ray Laue photos were taken on both sides and in the middle of the cloudy region along the KCl &lt;100&gt; direction to confirm the existence of the two TlI orientations.
Discrete spots observed on the Laue photos were evidence of the epitaxial growth of TlI on KCl. Further X-ray analysis details this epitaxy and provides a model for the observed microstructures.
In every case analyzed the TlI b.sub.o axis is aligned parallel to a KCl &lt;110&gt; direction. This suggests the importance of the KCl &lt;110&gt; direction that was noted under polarized light. Observation of orientations away from the principle orientations reveals that the TlI b.sub.o axis aligns itself with the KCl &lt;110&gt; direction nearest the substrate surface. Recalling the lens shaped crystal we realize that this alignment is the cause of the two orientations and their intersection along the &lt;100&gt; projection on the lens. This intersection has both KCl &lt;110&gt; directions equally near the surface. On exact KCl (100), which includes cleaved KCl, both &lt;110&gt; directions are in the surface plane and films on these surfaces contain the duplex microstructure and are cloudy.
In the present invention for vacuum depositing of low absorption transparent thallium iodide film on potassium chloride we have been able to eliminate the tendency for epitaxial growth to occur as a solution to the problem of cloudy coating. We have found that when TlI is condensed in its cubic (.beta.) phase on KCl and allowed to transform to its stable orthorhombic (.alpha.) phase a clear, polycrystalline, randomly oriented film develops independent of substrate orientation.
The condensation of the .beta. phase is accomplished by holding the substrate at 140.degree. C. or above during the deposition. The phase change occurs during substrate cooling with the grain size being dependent on the condensation temperature and cooling rate.
At elevated substrate temperatures and low deposition rates, however, TlI will not condense on the KCl. Use of higher evaporation rates solves this problem. By using a graphite Knudsen style evaporation source we have achieved more uniform heating of the TlI and are able to successfully deposit films at rates over 3000A per minute.
Films prepared in this manner at an arbitrary rate of 2000 A/min. are not cloudy, have low absorptions, and have withstood the laser damage tests.
This fabrication technique is especially compatible with the subsequent high temperature deposition of low index coatings.